For instance, in a multi-cylinder diesel engine or a multi-cylinder gas engine for power generation, if misfire takes place in a cylinder or a plurality of cylinders, an engine output is lowered to an output at which stable operation is possible simultaneously with detection of misfire by a misfire-detection unit, in order to continue stable operation of the engine.
Specifically, in a conventional multi-cylinder engine, when all cylinders are in a normal operation state and operating at the 100% output, if misfire occurs in two of the cylinders, the operation output level is lowered to a 50% output (90% output in a case of one cylinder) to operate the engine stably.
When misfire occurs in one or two cylinders, the torsional response amplitude of a crank shaft of an engine changes, and the aspect of the change in the torsional response amplitude is varied between the misfiring cylinders. Thus, the allowable maximum load of the engine due to misfire is varied between the misfire-occurring cylinders.
Thus, to optimize the operation output of an engine upon occurrence of misfire, it is important to evaluate and study the torsional response amplitude of the crank shaft.
Various evaluations and studies on torsional vibration have been provided as follows.
For instance, Non-patent Document 1 describes that torsional vibration is caused by rotation weights of a crank shaft which is a rotational shafting system, and there is a certain natural frequency depending on the strength of the shaft and the distribution condition of the rotation weights (Holzer method).
For instance, in a case where a shaft has N rotation weights, there are (N−1) natural frequencies having one node, two nodes, three nodes, . . . and (N−1) nodes. Here, one node means that the vibration has one node, and x nodes means that there are x nodes.
When the number of cycles of a vibratory force which causes the torsional vibration is the same as the natural frequency having x nodes, torsional vibration is caused by resonance.
The vibratory force is generated by a component-force vector, which is a sine wave vector obtained by analyzing a torque curve of an engine with a harmonic analyzer. Thus, torsional vibration appears as y-order torsional vibration with x nodes, determined by x, which is the number of nodes of vibration, and y, which is the order of the harmonic component-force vector that becomes the vibratory force.
Generally, when a relationship between a torque T and a torsional angle θ is explained referring to a shaft having a length L as a fixed end for fixing one end, the following equation is satisfied, in which a node point is the fixed end.θ=T×L/G×Ip  (1)
Here, T is a torque applied to the free end, G is a transverse elasticity coefficient of a material, and Ip is the polar moment of inertia of area with respect to an axial center.
According to the above equation, the torsional angle is proportional to the amplitude of the vibration. At each point of the shafting system, the amount of torsion of the shaft due to vector Ay of the harmonic component force is proportional to TL. Thus, a product of the harmonic vector Ay of each cylinder and a distance between the above node point and the cylinder is proportional to the total torsional angle of the shaft. That is, the magnitude of ΣAy×L is proportional to the amplitude of the torsional vibration.
Here, each vector Ay has a phase between the respective cylinders. Thus, ΣAy×L can be calculated by a graphical method using a TL vector chart.
The TL vector varies depending on the ignition order in a multi-cylinder engine. That is, if the crank arrangement and the ignition order of an engine are changed, the proportion magnitude CTL of the TL sum vector would become considerably different.
According to a result of performing harmonic analysis on a rotational-force torque T caused by one cylinder, the value of the harmonic component force Ay is varied depending on the order y. When the proportional magnitude here is CA, the vibratory force Cv of the vibration is:Cv=CA×CTL. 
The amplitude of the torsional vibration is determined in proportion to Cv. By calculating Cv continuously, the magnitude of the vibration that should appear for each order of the torsional vibration can be predicted.
As described above, it is necessary to have advantageous ignition timing and crank arrangement on the basis of prediction of the torsional vibration that should appear in the shafting system. Since an ignition timing and a crank arrangement have a significant relationship with balancing of an engine, it is necessary to determine the most advantageous crank arrangement and ignition timing in view of both of the prediction of the torsional vibration and balancing.
Further, in Non-patent Document 2, the following study has been conducted on the torsional vibration during misfire of an engine with a five bladed propeller and six cylinders.
Here, among the methods for calculating torsional vibration response, a simulation calculation method to which a steady-state vibration method is applied is used to evaluate vibration and torsional vibration stress at a resonance rotation speed of a torque harmonic order. The characteristics of torsional vibration during misfire and the interaction between the engine vibratory force and the propeller vibratory force are evaluated through examples. The process will not be described here, and only the result of the study will be shown below.
For example, when misfire occurs in one cylinder in an engine equipped with six cylinders, the fourth, fifth, and sixth torque harmonics increase. As a result, the fourth and fifth components, which have small torsional vibration stress in normal ignition, increase. The increase of the torsional vibration stress of the fourth component is especially remarkable, and may exceed the predetermined allowable stress curve in some cases.
Thus, the applicant of the present invention discloses a method and a system for controlling a load during misfire of an engine in Patent Document 1, whereby it is possible to improve the availability of the engine upon occurrence of misfire by enabling setting the allowable maximum load on an engine during occurrence of misfire to be a suitable value for each cylinder in which misfire is occurring when misfire is occurring in one cylinder or a plurality of cylinders.
Further, in Patent Document 2, the applicant of the present invention proposes a method and a device for restricting a decrease in availability of an engine during occurrence of a misfire and restricting a decrease in efficiency of an engine power generation plant that accompanies deterioration in the fuel consumption rate of the engine.
Specifically, proposed here is a method and a system for performing output limit operation of an engine on the basis of a detection result of misfire of an engine equipped with a plurality of cylinders. On the basis of a detection signal of misfire, the first limit output, which is an output obtained by subtracting an output due to misfire corresponding to the number of cylinders with misfire from an output in normal operation, is calculated. Also, the second limit output is calculated on the basis of the detection signal of misfire using an output limit value that is set on the basis of a relationship of a change in torsional vibration and a cylinder with misfire that is set in advance. Then, the first limit output and the second limit output are compared to calculate the minimum limit output, and the engine is operated having the minimum limit output as the allowable maximum output during misfire.
As a result, a suitable output limit rate is determined so that the utilization rate of the engine decreases and it is possible to operate an engine at a low output that is beyond necessity, while the output of the engine is uniformly reduced by 50% in a conventional case when misfire is occurring in one or two cylinders.